Condensing air preheater with heat pipes

ABSTRACT

An air preheater comprises flow conduits for heat transfer between a heating gas and combustion air which is to be pre-heated with heat pipes extending between the flow conduits for the two gas streams to provide heat transfer The fins on the heat pipes in at least the condensation zone of the flow conduit of the heating gas are provided with one or more serrations on the sides of the pipes facing away from the oncoming gas flow. The serrations assist in removing any liquid film which may accumulate on the fins during operation in the condensing mode. A corrosion resistant coating material such as glass may be provided on the condensing side of the finned heat pipes.

This invention concerns a condensing air preheater system that utilizes finned heat pipes for effecting the heat transfer.

BACKGROUND OF THE INVENTION

Waste heat recovery units are used to capture additional heat from the flue gas as it leaves boilers and fired heaters. Air preheaters and economizers comprise one type of heat recovery unit and have the capability to increase the capture of the latent heat in the flue gas but their increased heat recovery potential is limited by working conditions which restrict condensation of the flue gas.

Preheaters such as these may operate in condensing or non-condensing mode. In the condensing mode, the heating fluid is initially in the vapor state and during its passage through the exchanger but when it gives up heat to the cooler combustion air, condensation from the vapor state takes place forming a liquid phase in the stream. A typical instance of this is with heat exchangers using flue gases from a combustion process in a furnace or a boiler as the heat source; as these exhaust streams will contain not only nitrogen from the original combustion air and carbon dioxide from the combustion process but also water from the combustion of hydrocarbons, the liquid phase which forms on cooling will be water which will form on and around the heat exchange elements in the exchanger. Not only does this liquid phase impede good heat transfer but since acid components including carbon dioxide and, often, sulfur oxides, it is also frequently corrosive towards the metal components of the exchanger.

Condensing air preheaters and economizers using a heat transfer surface to condense water vapor from the flue gases are inherently more efficient in their potential to realize an enhanced recovery of the waste heat. While there are inherent advantages with these heat exchangers, they come at a price as the condensate is usually laden with corrosive constituents such as sulfuric acid. The design and materials of construction of these types of equipment is therefore of the utmost importance for optimal performance and reliability.

Tubular and heat pipe heat exchangers are examples of this sort of equipment that operate in a condensing environment. From an operational perspective, their design is favored by an increased surface area as this increases the heat transfer potential, i.e. greater surface area translates into a larger contact surface both between the flue gas and the transfer components and between the transfer components and the cool fluid on the other side of the exchanger; the heat transfer area can be optimally increased with minimal contributions to the overall size of the heat exchanger by the use of finned tubes.

The transfer of heat between the two streams is also susceptible to improvement by the use of heat pipes extending between the two sides of the exchanger. Heat pipe air preheaters consist essentially of a bundle of self-contained heat pipes. Each heat pipe is partially filled with a working fluid, most commonly water or hydrocarbon, and sealed. In a typical design, the heat pipes are arranged in an array of parallel rows, attached at their respective midpoints to a divider plate which both supports the pipes and provides a barrier between the flue gas and combustion air which typically pass in countercurrent flow to maximize heat transfer between them. Heat from flue gas, for example, evaporates the working fluid collected in the lower or evaporator end of the pipe and the vapor flows to the upper or condenser end of the pipe located in the other half of the exchanger where it gives up heat to the incoming combustion air. Condensed fluid returns by gravity to the evaporator end. The process continues indefinitely as long as there is a temperature difference between the flue gas and the combustion air.

Combustion air preheaters using heat pipes to effect the heat transfer between the hot exhaust gases to the cooler combustion air are described, for example, in U.S. Pat. No. 5,085,270 (Counterman). This patent describes a preheater which has a number of finned heat pipes arranged in parallel, superposed rows with a divider plate providing a barrier between a flue gas stream and a counterflowing combustion air stream.

Currently, air preheaters with heat pipes are designed for non-condensing operation based on two significant considerations. First, no cost-effective way is available to protect the heat transfer surface from corrosion from acidic condensate without a reduction in the heat transfer. The current options for corrosion protection are to fabricate the heat pipes with expensive corrosion resistant materials or to cover them with the corrosion resistant film such as Teflon™ or enamel coatings. Employing expensive corrosion resistant materials is cost-prohibitive. Covering the surface with a corrosion-resistant coating such as a perfluorocarbon e.g. Teflon™ film makes it impossible to attach fins outside the heat pipes, which is the most common technique to enhance heat transfer for the current non-condensing units. Enamel coatings are being used to protect a part of air preheaters from cold end corrosion, but are applied to bare tubes, not finned tubes. Second, eliminating condensate is crucial in improving the performance of the condensing air preheater but it is difficult to remove condensate from the finned heat pipe surfaces of current air preheaters. The liquid condensate layer creates a resistance to heat transfer due to its low thermal conductivity. With horizontal heat pipes, a thick condensate layer forms on the lower portion of tubes and with vertical finned heat pipes, the back side of the pipe can become flooded by condensate. Consequently, these two issues need to be resolved if finned heat pipes are to be applied to condensing air preheaters.

SUMMARY OF THE INVENTION

We have now developed an improved condensing mode air preheater with heat pipes in which the heat recovery rate from the heating gas is substantially improved. Operation in the condensing mode implies, of course, that the heating gas comprises partly or exclusively a component which is susceptible to condensation under the conditions encountered by it in the preheater, i.e. when heat is transferred out of the gas by the heat pipes into the combustion air which is being heated. Flue gas is a particular example of such a heating gas as it contains water vapor from the combustion process; waste steam may be another. Condensation is most likely to occur in the condensation region or section at the end of the heat pipe array where the heating gas has been cooled to its greatest extent. According to the invention, serrated fins and, optionally, coatings are used on the heat pipes in the section where condensation occurs to improve heat transfer and to prevent corrosion; they may suitably be used on all the pipes in the entire heat pipe array if desired. The serrations may be imposed selectively on certain portions of the heat pipe circumference or, alternatively, around the entire periphery of each pipe. Another possibility is that fins with selectively located serrations may be used in certain areas and completely serrated fins in other locations.

According to the present invention, the air preheater comprises: a first flow conduit for a heating gas, a second flow conduit in heat transfer communication with the first flow conduit for a gas to be heated, heat pipes extending from the first flow conduit into the second flow conduit to provide heat transfer communication, the heat pipes having: an evaporation zone located in the first flow conduit, a condensation zone located in the second conduit, and fins extending out from the heat pipes into the respective flow conduits, the fins on the heat pipes in at least the first flow conduit having at least one serration on the sides of the pipes facing away from the oncoming gas flow.

The heating gas flowing in the first conduit is, as pointed out above, particularly susceptible to condensation with consequent formation of liquid films in the region of the conduit where the heating gas reaches its lowest temperatures, namely, the condensation section of the conduit. The serrated fins are primarily located in this region as this is where the problems described above arise. The serrated fins may be provided exclusively on the heat pipes located in the condensation section or, optionally, on all the pipes in the first conduit. The preheater can therefore be regarded in more detail as comprising: a first flow conduit for a heating gas comprising a vapor component which is susceptible to condensation upon cooling in the preheater, the first flow conduit in the preheater having a condensation region in which condensation of the vapor component of the heating gas occurs during operation in the condensing mode; a second flow conduit in heat transfer communication with the first flow conduit for a gas to be heated, heat pipes extending from the first flow conduit into the second flow conduit to provide heat transfer communication, the heat pipes having: an evaporation zone located in the first flow conduit, a condensation zone located in the second conduit, and fins on the heat pipes which extend out from the heat pipes into the respective flow conduits, the fins on the heat pipes in at least the first flow conduit having at least one serration in the fins on the sides of the pipes facing away from the oncoming gas flow.

Further features and constructional options are described below.

DRAWINGS

FIG. 1 is a vertical section of an air preheater with finned heat pipes;

FIG. 2A is a cross section of a heat pipe with a circular fin with one serration on its downstream side;

FIG. 2B is a cross section of a heat pipe with a circular fin with three serrations on its downstream side; and

FIG. 3 is an isometric drawing of a sectioned heat pipe with fins serrated around the entire circumference of the pipe.

DETAILED DESCRIPTION

Heat pipe air preheaters essentially consist of a bundle of self-contained heat pipes which act to transfer heat from a fluid providing the heat source to the air which is to be heated as a heat sink. In a typical design, heat pipes are arranged in an array with rows of pipes which extend transversely to the directions of fluid flow in each conduit. A number of transverse rows are positioned at successive locations along the gas flow paths of the two respective conduits. The heat pipes are attached at their respective midpoints to a divider plate which both supports the pipes and provides a barrier between the flue gas and combustion air which usually flow in countercurrent to one another to optimize heat transfer. Each heat pipe is partly filled with a working fluid, most commonly water or hydrocarbon, and sealed. The warm flue gas transfers heat to the evaporator ends of the heat pipes to evaporate the working fluid and the heated vapor flows to the other, condenser end, where it gives up heat to the incoming combustion air flowing over the condenser ends of the pipes.

The basic components of a heat pipe are the container and the working fluid, optionally with a wick or capillary structure. The function of the container is to isolate the working fluid from the outside environment. It has to therefore be capable of maintaining a pressure differential across its walls and a high thermal conductivity to enable transfer of heat to take place from and into the working fluid with minimum temperature drop across the walls of the container. Typical container materials are copper, nickel, aluminum and aluminum alloys.

The working fluid within the container is selected to have a vapor temperature range appropriate to the intended operations. The vapor pressure over the operating temperature range must be sufficiently great to avoid high vapor velocities, which tend to setup large temperature gradients and cause flow instabilities. The fluid should exhibit good thermal stability, a vapor pressure not too high or low over the operating temperature range a high latent heat, high thermal conductivity, low liquid and vapor viscosities and acceptable freezing or pour point. A high value of surface tension is desirable in order to enable the heat pipe to operate against gravity and to generate a high capillary driving force with a small contact angle so as to wet the wick (if present) and the container material. The selection should also be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe such as viscous, sonic, capillary, entrainment and nucleate boiling levels.

Typical working fluids which may be suitable in the present air preheater application include, for example, acetone and other ethers, alcohols such as methanol, ethanol, propanol, butanol, hydrocarbons such as toluene, perhalocarbons, water, Mercury is normally excluded for environmental reasons although possibly otherwise suitable. Liquid metals such as sodium, lithium and sodium/potassium alloy offer the possibility of high temperature application but are not usually required in the present service.

While a wicking material is not required with the vertical pipe positioning, it is not excluded although it may create a complexity in design. The prime purpose of the wick is to generate capillary pressure to transport the working fluid from the condenser to the evaporator. It must also be able to distribute the liquid around the evaporator section to any area where heat is likely to be received by the heat pipe. These two functions may require wicks of different forms with the selection of wick depending on various factors, several of which are linked to the properties of the working fluid. The maximum capillary head generated by a wick, for example, increases with decreasing pore size while wick permeability increases with increasing pore size, requiring a balance to be struck between these two factors with the pore sizing appropriate to the selected values. Another feature of the wick to be optimized, is its thickness with the heat transport capability of the heat pipe being raised by increasing the wick thickness.

Wick material has a porous structure and is typically made of materials like steel, aluminum, nickel or copper in various ranges of pore sizes. They are fabricated using metal foams, felts and sintered powders. Sintered powders provide high thermal transfer capacity, low temperature gradients and high capillary forces for anti-gravity applications. Screen mesh is used in many the products and provides readily variable characteristics in terms of heat transport and orientation sensitivity, according to the number of layers and mesh counts used Fibrous ceramics have also been used although they have little stiffness and usually require a support by a metal mesh. More recently, interest has turned to carbon fibers as a wick material. Carbon fiber filaments have many fine longitudinal grooves on their surface, have high capillary pressures and are chemically stable. The small capillary driving force generated by the axial grooves is adequate for low power heat pipes when operated horizontally, or with gravity assistance. The tube can be readily bent and when used in conjunction with screen mesh the performance can be considerably enhanced.

The heat pipes are preferably arranged vertically with the evaporator end at the lower location and the condenser at the upper location so that the fluid which condenses at the cooler end will return by gravity to the warmer evaporator end. The process continues indefinitely as long as there is a temperature difference between the flue gas and the combustion air. Horizontal or inclined pipes may however be used although their use is less favored as a wicking material needs to be provided to promote movement of the condensed material back from the condenser end to the evaporator end. An inclination of generally 5 to 15 degrees from the horizontal will offer advantages in efficiency over a completely horizontal position. The capacity of the individual heat pipe depends upon several factors, including its inclination angle and the temperature differential between its ends, increasing both as the inclination angle and the temperature differential increase.

As shown in FIG. 1, the preferred design utilizes vertical heat pipes with external fins. The preheater 10 has a conduit 11 its lower section through which the high temperature fluid, which is the flue gas, flows from inlet end 14 to outlet end 16. A dividing wall 12 separates the lower conduit 11 from the conduit 13 in the upper section of the preheater in which the low temperature fluid, which is combustion air, flows from inlet end 19 to outlet end 17 in countercurrent fashion to the flow of the flue gas in the lower section. Heat pipes 15 are arranged in an array of plural parallel rows extending across the gas flow paths with successive rows arranged along the gas flow paths in the conduits. The heat pipes may be fixed in either the triangular or square configurations characteristic of heat exchangers in order to maximize contact between the two gases and the heat pipes. The pipes are fixed at approximately their midpoint in dividing wall 12 so as to extend from the lower conduit 11 into the upper conduit 13 to effect heat transfer from the warm flue gas to the cool combustion air. The heat pipes have the construction described generally above with a working fluid of selected appropriate characteristics within the container and optionally, a wicking material. The vertical disposition of the heat pipes may eliminate the need for the wick material because the condensed working fluid formed in the upper low temperature condensation zone will fall gravitationally to the high temperature (lower) zone of the pipe. The design is then simplified with a reduction in manufacturing cost. The flue gas passes from the outlet 16 of the lower conduit 11 to stack or to treatment, e.g. scrubbing, before discharge to stack. The combustion air, heated by its passage through the preheater, passes from outlet 17 of the upper conduit to the burner of the furnace, boiler or other fired equipment.

In its passage through the conduit and past the heat pipes, the flue gas is cooled as heat is transferred to the heat pipes and from them, into the combustion air on the other side of the dividing wall. As the flue gas is progressively cooled by its passage over the heat pipes, the potential for condensation of the water in it increases as its temperature decreases with condensation taking place in the condensation section 18 towards the outlet end of the lower conduit. It is in this condensation section of the flue gas conduit that the serrated fin heat pipes are used to reduce the problems which occur with operation in the condensing mode. The serrated fins may be used on an exclusive basis in this section of the conduit or, alternatively, on all the heat pipes in the array as a whole.

Circular fins are attached on both the upper 20 and lower sections 21 of heat pipes 15 and extend out from the body of the pipe into the gas flow path in the conduit to assist heat transfer between the two respective fluids and the heat pipes. As shown in FIG. 2A, a single serration is provided on the downstream (back) side 25 of fins in the lower section; FIG. 2B shows a section of a heat pipe with three serrations 26 a, 26 b, 26 c on the back or downstream side (downstream is determined by reference to the direction of flow of the gas over the pipe with the downstream facing away from the direction of the oncoming gas flow as indicated by arrows 27, 28, respectively). The purpose of the serrations on the fins is to enable condensate to be removed efficiently by the gas flow in the lower conduit so enhancing heat transfer by the removal of the insulating liquid layer on the finds. These serrations can be fabricated with a milling machine after attaching the fins to the containers of the heat pipes. Since the purpose of the serrations is to remove condensation, the serrated fins may be provided at least or exclusively on the heat pipes in the area where condensation occurs in the lower conduit when operating in the condensation mode. There is, however, no prejudice, apart from cost considerations, to the use of serrations on all the heat pipes in the lower conduit. Since condensation is not a problem in the upper conduit, it is not necessary to provide serrations on the fins in the upper evaporation zone although they may be provided in that zone to improve heat transfer to the combustion air by increasing access by the air to the hotter portion of the fins in the region immediately next to the containers of the heat pipes. The fins on the tubes typically have a height (root-to-tip) of about 1.3 cm with an overall thickness of about 1 mm with 2 fins/cm. These dimensions may be varied according to individual requirements and manufacturing convenience and cost although with possible changes to heat transfer efficiency.

To provide protection against corrosion in the condensation zone, a corrosion-resisting coating material is applied to the heat pipe and finned surface in this zone. The coating material may, for example, by enamel, a perfluorocarbon e.g. Teflon, or a polyphenylene sulfide (PPS)-based material, e.g. Ryton™ (Chevron Phillips) or Fortron™ (Ticona). With the use of the corrosion-resisting coating it is possible to fabricate the heat pipe with cost-effective materials, including carbon steel, copper, or aluminum, with reduced risk of corrosion. The coating materials can be mixed with other materials such as carbon nano tube or carbon fiber, to further improve the thermal and mechanical performance of the heat pipes.

Corrosion resistant glass coating is a preferred coating material, being favored over other coating/lining materials for its ability to withstand strong acids at elevated temperatures (i.e. dew point temperature of the flue gas). Glass is also known to be inert, thereby eliminating any potential of cross-contaminating the condensate and increasing any potential adverse environmental effects. The glass coating will also provide a smooth surface which will increase the rate of condensate coming off the surface of each tube.

The glass coating is a surface treatment specifically selected for its ability to adhere onto the metallurgy of the heat exchanger and to withstand the operating environment of the heat exchanger. For the glass to perform as expected, it needs to be applied in a controlled environment following carefully prescribed operating procedures and quality control practices. A critical element in the application of the glass coating is to round off any sharp edges on the surface of the finned tubes. This assists in the overall application of the coating as the glass needs to be 100% pinhole-free before being put into service.

One variation is to make use of serrated fins as an alternative to circular fins as shown in FIG. 3 where the fins 30 (only one indicated) are arranged helically around the central heat pipe. Serrated fins more effectively promote condensate removal, and at the same time provide additional enhancement to heat transfer. If additional milling is required to form the serrations around the entire periphery of the fins, this configuration may be more expensive to fabricate, when compared to the simpler variant with a limited number of serrations on the back side of the pipes as shown in FIGS. 2A and 2B although pipes with continuously serrated helical fins as shown in FIG. 3 may be fabricated by winding a serrated strip onto the exterior of the pipe and fastening the strip onto the pipe by soldering or welding or, alternatively, by mechanically embedding the strip in a groove cut into the outer diameter of the tube and locking the strip in place by rolling to force the groove to close tightly around the base of the fin. A coating material is desirable in this case also to protect the heat transfer surfaces from corrosion. 

1. An air preheater comprising: a first flow conduit for a heating gas, a second flow conduit in heat transfer communication with the first flow conduit for a gas to be heated, heat pipes extending from the first flow conduit into the second flow conduit, the heat pipes having: an evaporation zone located in the first flow conduit, a condensation zone located in the second conduit, and fins on the heat pipes which extend out from the heat pipes into the respective flow conduits, the fins on the heat pipes in at least the first flow conduit having at least one serration in the fins on the sides of the pipes facing away from the oncoming gas flow.
 2. An air preheater according to claim 1 in which the fins on the heat pipes in at least the first flow conduit have a plurality of serrations on the sides of the pipes facing away from the oncoming gas flow.
 3. An air preheater according to claim 1 in which the fins on the heat pipes in at least the first flow conduit have two or three serrations on the sides of the pipes facing away from the oncoming gas flow.
 4. An air preheater according to claim 1 in which the fins on the heat pipes in at least the first flow conduit have a plurality of serrations extending around the entire periphery of the pipes.
 5. An air preheater according to claim 1 in which serrated fins extend out from the heat pipes into both the first and second flow conduits, with at least one serration in the fins on the sides of the pipes facing away from the oncoming gas flow.
 6. An air preheater comprising: a first flow conduit for a heating gas comprising a vapor component which is susceptible to condensation upon cooling in the preheater, the first flow conduit in the preheater having a condensation region in which condensation of the vapor component of the heating gas is likely to occur; a second flow conduit in heat transfer communication with the first flow conduit for a gas to be heated, heat pipes extending from the first flow conduit into the second flow conduit, the heat pipes having: an evaporation zone located in the first flow conduit, a condensation zone located in the second conduit, and fins on the heat pipes which extend out from the heat pipes into the respective flow conduits, the fins on the heat pipes in at least the first flow conduit having at least one serration in the fins on the sides of the pipes facing away from the oncoming gas flow.
 7. An air preheater according to claim 6 in which the fins on the heat pipes in the condensation region of the first flow conduit in have a plurality of serrations on the sides of the pipes facing away from the oncoming gas flow.
 8. An air preheater according to claim 7 in which the fins on the heat pipes in the condensation region of the first flow conduit have two or three serrations on the sides of the pipes facing away from the oncoming gas flow.
 9. An air preheater according to claim 6 in which only the fins on the heat pipes in the condensation region of the first flow conduit in have a plurality of serrations on the sides of the pipes facing away from the oncoming gas flow.
 10. An air preheater according to claim 9 in which only the fins on the heat pipes in the condensation region of the first flow conduit have two or three serrations on the sides of the pipes facing away from the oncoming gas flow.
 11. An air preheater according to claim 6 in which the fins on the heat pipes in the second flow conduit have a plurality of serrations extending around the entire periphery of the pipes.
 12. An air preheater according to claim 6 in which serrated fins extend out from the heat pipes into both the first and second flow conduits, with at least one serration in the fins on the sides of the pipes facing away from the oncoming gas flow.
 13. An air preheater according to claim 6 in which the fins are helical fins on the heat pipes.
 14. An air preheater according to claim 11 in which the fins are helical fins on the heat pipes.
 15. An air preheater according to claim 12 in which the fins are helical fins on the heat pipes. 